testing od Pile inclination

1. Testing of bored pile inclination
Joram M. AMIR
Chairman, Piletest.com Ltd. JMAmir@Piletest.com
Erez I. AMIR
President, Piletest.com Ltd. Erezam@piletest.com
Keywords: deep foundations, inclination, testing, raked piles
ABSTRACT:
The allowable deviation from of piles from verticality is mentioned in practically all piling specifications,
with typical values ranging from 1.33 to 2 percent. Similarly, specifications also limit the tolerance of raked
piles from their specified inclination. While this restriction seems critical for piled retaining walls, the
reasoning behind this restriction for piled foundations is not well understood. Finite element simulation we
carried out has shown that exceeding the above limits can introduce large bending moments and shear forces
in piles designed strictly for axial loads and may even lead to structural failure. Still, the above specifications
are seldom enforced due to the lack of convenient testing apparati. In this paper, we describe the BIT
(Borehole Inclination Tester), present its details and calculation method, and show initial field results.
1 INTRODUCTION
Probably all piling specifications prescribe the
tolerances for both centricity and verticality of
piles. The following examples demonstrate
common values for allowable deviation:
“Verticality – the maximum permitted deviation
of the finished pile from the vertical at any level is
1 in 75. Rake – the maximum permitted deviation
of any part of the finished pile from the specified
rake is 1 in 25 for piles raking up to 1 in 6 and 1 in
15 for piles raking more than 1 in 6” (ICE 1996).
“The vertical alignment of a vertical shaft
excavation shall not vary from the plan alignment
by more than 20 mm per meter (1/4 inch per foot)
of depth. The alignment of a battered shaft
excavation shall not vary by more than 40 mm per
meter (1/2 inch per foot) of the distance along the
axis of the shaft from the prescribed batter”
(O’Neill and Reese 1999).
The importance of controlling the verticality of
a contiguous piled retaining wall is clear: piles
protruding excessively into an excavation for a
parking basement, for instance, may cause the loss
of parking spaces and hinder the granting of a
building permit. Strict verticality is even more
important in secant pile walls, where misalignment
can make the wall non-watertight (Fleming et al.
1994). The questions we set out to answer were
twofold.
• Why is verticality important for single
foundation piles?
• How can we quickly check conformance of
bored piles to the specifications regarding
inclination?
In the following, we shall use finite element
simulation to prove the importance of complying
with such specifications. Later on we shall present
a brief overview of existing devices which can
measure pile verticality and then describe the
system which we developed specifically for testing
pile inclination. Initial field results which we have
obtained demonstrate the viability and accuracy of
this technique.
2 TERMINOLOGY
Borehole axis – The trajectory the Kelly bar tip
follows when lowered into the borehole.
Inclination – the angle, in either degrees or
percent, between the vertical and the borehole axis
at any given depth.

2. Battered pile or raking pile – a pile purposely
constructed at an inclination.
Stop – a depth in the hole at which stabilized
inclination readings are taken.
Depth interval – the mean distance along the
borehole axis between two consecutive stops.
Deviation – the vector, measured in the
horizontal plane, from the planned borehole axis to
the actual one.
3 FINITE ELEMENT SIMULATION
To get a better understanding of the effect of
excessive inclination, we ran a two-dimensional
Plaxis simulation of a diaphragm wall 600 mm
thick and 20 m deep, embedded in soft clay. The
wall inclination was 2.5 percent from the vertical,
a mere 0.5 percent above the typically acceptable
value. We checked two cases: a free wall and a
horizontally-restrained one.
The results show the distribution of normal
force N, shear force Q and bending moment M
down the wall for both a free-headed (Figure 1)
and a fixed-headed (
Figure 2) wall. It shows clearly that even a small
excessive deviation can introduce large,
not-accounted for, shear forces and bending
moments in free walls. In restrained walls, e.g.
those connected by beams, this effect is minimal.
Although this two-dimensional simulation is not
fully-valid for three dimensional piles, the results
are at least qualitatively applicable.
Figure 1: Effect of vertical load on inclined
free-headed wall
Figure 2: Effect of vertical load on inclined restrained wall
4 EXISTING SYSTEMS
Present methods for measuring the inclination of
bored piles are invariably based on the plumb bob
principle: the instrument is hung down from the
planned center of the pile, and the distance to the
sidewalls is measured by ultrasonic sensors (Bruce
et al. 1989, Kort and Kostaschuk 2007). The
sensors are arranged in either of two fashions:
• Four transceivers in two mutually-
perpendicular sets, providing four readings per
every depth
• One rotating transceiver, giving 360º coverage.
Once the hole is completely logged, the
sidewall profile is drawn and, provided it is
relatively regular, the pile axis can be plotted and
compared to the vertical. A bonus of this approach
is that it enables the calculation of the bore
volume, an item of special interest to contractors.
An interesting and simple method, described by
Bruce et al. (1989), is the inverted pendulum in
which a buoy floating on the slurry replaced the
plumb bob.
The above systems have several drawbacks:
• They can be used only in slurry-filled holes
• They cannot be used in slender piles (when the
slenderness ratio L/d exceeds 25, a deviation of
2% or more from the vertical cannot be
measured)
• They cannot be used to measure the inclination
of raked piles
• They cannot be used to measure the inclination
of finished piles
• Some of these systems are cumbersome and
need a dedicated crane to move around the site
and an AC generator to power them.
5 THE PROPOSED SYSTEM
5.1 Description
The system is dual purpose:
• During drilling, it can be mounted on the
drilling bucket that acts as centralizer.
• In the finished pile, it can be lowered inside the
access tubes which are routinely installed for
crosshole ultrasonic testing (ASTM 2008).
The system consists of four main units (Figure
3):

3. 1. The upper unit which contains the electronic
circuitry and is mounted inside a 100 m.
cable-reel
2. The lower unit, containing a bi-axial (X-Y)
Micro Electro-Mechanical System (MEMS)
inclinometer, a MEMS gyro and a
thermometer, all packaged in a compact
waterproof housing. For drilling monitoring,
the lower unit is rigidly mounted on the drilling
bucket crossbar while for finished pile testing it
is mounted on a stabilizer that acts as both a
centralizer and a rotation-suppressor.
3. A wireless depth meter
4. A hand-held computer for recording and
presenting data
Figure 3: Schematic drawing of the system components
The lower unit transmits the data to the upper
one via a cable that runs over the depth meter
wheel and operates it. The upper unit uses wireless
communication to both the depth meter and the
computer.
The actual system components are depicted in
Figure 4.
5.2 Operation - Drilling configuration
After the hole is drilled to the desired depth, the
lower unit is fixed on the bucket cross bar and
inserted into the hole while pointing to the
reference north. The inclination and azimuth are
measured at ground level and then at
predetermined stops until the drilling bucket
reaches the bottom of the hole. The procedure is
then repeated on the way up, with readings taken at
the same stops as before. During the whole
procedure, the rig operator should keep Kelly bar
rotation to a minimum (although some degree of
unavoidable rotation is tolerable). When the bucket
returns to the surface, it is aligned back with the
reference north, and a final reading taken. Typical
test duration is in the order of 10 to 15 minutes.
5.3 Operation - Access tube configuration
The lower unit with the stabilizer is inserted
into the access tube pointing to the reference north.
The inclination and azimuth are measured at pile
head level and then at predetermined stops until
the unit reaches the bottom of the tube. The
procedure is then repeated on the way up, with
readings taken at the same stops as before. When
the unit returns to the surface, it is aligned back
with the reference north, and a final reading taken.
Figure 4: The BIT system
5.4 Calculations (Figure 5)
1. For each stop numbered [n], with sensor
heading hn, the inclinations Xn and Yn in the X
and Y directions, respectively, are transformed
to inclinations in the North and East directions
by the following relationships.
)sin(h·Y-)cos(h·XIncEast nnnnn += (1)
)cos(h·Y-)sin(h·X-IncNorth nnnnn = (2)
2. For each stop numbered [n] at depth = Dn, the
deviations of the pile axis in the N-E directions
are calculated as follows.

4. 0DevEast1 = (3)
0DevNorth1 = (4)
)
2
tan()(
DevEastDevEast
1
1
1-nn
−
−
+
⋅−
+=
nn
nn
IncEastIncEast
DD
(5)
)
2
tan()(
DevNorthDevNorth
1
1
1-nn
−
−
+
⋅−
+=
nn
nn
IncNorthIncNorth
DD
(6)
Figure 5: Inclination and deviation explained
And the combined deviation expressed as a polar
vector is:
nnn rDev θ∠= , (7)
where
22
nnn DevEastDevNorthr += (8)






=
n
n
n
DevNorth
DevEast
arctanθ
(9)
3. Ideally, for the final surface reading N, the
final calculated deviation rf should be zero.
4. If the closure error rf is less than a
pre-determined threshold value (expressed as a
fraction of pile length), it is proportionately
distributed among all stops.
5. If the closure error rn exceeds the threshold
value the operator receives a warning and may
decide to repeat the test.
6. For each stop, the corrected downward and
upward deviations are averaged and
transformed to polar coordinates.
5.5 Calibration
Since the specifications typically allow a deviation
in the order of only 1 to 2 percent, the system
accuracy should be at least one order of magnitude
smaller or better than 0.1%.
MEMS components are generally sensitive to
temperature changes. To achieve the required
accuracy, these components should be carefully
calibrated before the test, as follows:
Inclinometer calibration: The lower unit is
placed on a hard, smooth, horizontal surface and
stabilized inclination readings for both axes taken.
The lower unit is then rotated 1800
and the
procedure repeated. The mean X and Y readings,
respectively, are the level (or zero inclination)
readings.
Gyro calibration: MEMS gyro devices tend to
drift at a fairly constant rate. To compensate for
this drift the lower unit is laid stationary until the
rate of drift is constant, at which time it is
recorded.
Depth meter: A typical distance of 5 to 10 m is
marked on the cable and pulled over the depth
meter wheel.
5.6 Reporting
In addition to project and pile identification, the
report typically includes graphic presentation of
the results in two forms (Figure 6).
1. Top view of the pile axis superimposed on a
number of concentric circles showing the
radii in meters
2. Side view in which the maximum deviations
measured in all stops are connected by straight
lines on the background of a funnel depicting
the allowable deviation.

5. Figure 6: Typical results
6 CONCLUSIONS
Excessive inclination can be detrimental to piles
which were planned to be vertical, by introducing
large bending moments and shear forces. This
effect is much more prominent when the piles
heads are free to move.
The system presented is a portable instrument
for measuring the deviation of bored piles from the
vertical with accuracy within 0.1%. It may
therefore assist the geotechnical engineer to
enforce the specification for pile verticality. The
system may also serve piling contractors who have
to meet strict verticality tolerances in contiguous
and secant piled walls. In addition, it can help the
contractors to evaluate the suitability of specific
rigs to difficult site conditions, such as the
presence of boulders or rock.
7 REFERENCES
ASTM (2008). "Standard Test Method for Integrity Testing
of Concrete Deep Foundations by Ultrasonic Crosshole
Testing D6760-08", Vol. 04-09 pp. 943-949, West
Conshohocken PA
Bruce, D.A., Paoli, B.D., Mascardi, C. and Mongilardi, E.
(1989). “Monitoring and quality control of a 100 metre
deep diaphragm wall”, Proc. Intl. Conf. on Piling and
Deep Foundations.”, London,Vol 1, pp. 23-32.
O’Neill, M.W. and Reese, L.C. (1999). “Drilled shafts:
construction procedures and design methods”, ADSC,
Dallas, p. 448.
ICE (1996). “Specifications for piling and embedded
retaining walls”, Thomas Telford, London, p. 7.
Fleming, W.G.K., Weltman, A.J., Randolph, M.F. and Elson,
W.K. (1994). “Piling engineering (2nd ed.)” Blackie,
Glasgow, pp. 215-216.
Kort, D.A. and Kostaschuk, R.P. (2007). “Sonar calipering of
slurry constructed bored piles and the impact of pile
shape on measured capacity”, 8th Canadian Geotech.
Conf, Ottawa
O’Neill, M.W. and Reese, L.C. (1999). "Drilled Shafts:
Construction Procedures and Design Methods", US DOT
FHWA, Washington DC, p. 448